The present invention relates to a catalytic process for the preparation of carboxylic acids by catalytic cleavage of lactones over catalysts derived from metals of group VIII of the Periodic Table of the Elements. In contrast to the known cleavage of lactones by saporification, in which carboxylic acids having hydroxyl groups originating from the lactone group, or functions formed from these form, in the cleavage according to the invention a carboxyl function is formed from the lactone function present in the molecule, although no hydroxyl function or functional group derived therefrom is formed.
Lactones play an important role, for example, in the fragrance and flavor industry. By contrast, as a starting material for the preparation of other products, lactones have hitherto only appeared to a minor degree. This is due, firstly, to the reactivity of the lactones. In the simplest reaction of lactones, hydrolysis, hydroxycarboxylic acids or derivatives thereof are formed. It is generally necessary to reduce the hydroxyl function; firstly to provide carboxylic acids or also esters with a broad potential field of use, and secondly, of course, also to avoid the back-formation of lactone. The otherwise known reactions of lactones virtually always lead only to products which are of no or only low industrial importance and/or can be prepared more favorably by another method.
It is also a factor here that, in addition to the frequently undesired reactivity pattern, lactones are generally expensive products. This is due to the comparatively complex preparation process. Of the processes for lactone preparation, the telomerization of butadiene with CO2 and optionally further starting material compounds has recently become the focal point. As the starting materials are low in cost and present in large amounts, some lactones at least have become accessible at low cost and are available as starting substances for secondary reactions, some of which have still to be developed, to give products which are of potential interest for certain fields of application.
One example of a readily accessible lactone which may be mentioned is the xcex4-lactone 2-ethylidene-6-heptenolide, which is prepared in a telomerization reaction from two molecules of butadiene and one molecule of CO2. Organophosphine-modified palladium complexes are used as catalysts. The process can be modified in various ways. Using modem process variants, yields of 95% with regard to the lactone are possible, and even the problem of catalyst recycling has meanwhile been solved in a number of ways. According to the process disclosed in WO 98/57745, this is possible, for example, by immobilizing the catalyst on a polystyrene support, or by extracting the resulting lactone when the reaction is complete and returning the catalyst which is insoluble in the extractant. This last process is disclosed in Chemie-Ingenieur-Technik 72 (2000), pages 58 to 61.
Hitherto known secondary reactions which have been carried out on this lactone are alkaline cleavage, which produces, as product, 2-ethylidene-5-hydroxy-6-heptenoic acid, and acid-catalyzed cleavage in the presence of methanol, which produces a mixture of the methoxy derivatives and the methylester of the abovementioned acid, see Z. Chem. 25 (1985), pages 220 to 221. Also known is the synthesis of the corresponding xcex3-lactone from 2-ethylidene-6-heptenolide.
It is an object of the present invention to provide a process with which lactone cleavages can be carried out which produce, as secondary products, carboxylic acids which do not have hydroxyl groups or substituents derived therefrom. The process should be easy to carry out, produce high yields and leave intact other functional groups present on the lactone, such as, for example, olefin functions.
We have found that this object is achieved by a process for cleaving lactones optionally having functional groups to give carboxylic acids, this process comprising reacting the corresponding lactone with hydrogen under catalysis by a compound of a metal of group VIII of the Periodic Table of the Elements which has been modified with organophosphines.
The process according to the invention permits, as a result of the cleavage of lactones, the preparation of carboxylic acids which no longer have hydroxyl groups originating from the lactone function, but in which other functional groups are generally still present.
This is a transition-metal-catalyzed process in which catalytically active complexes of metals of group VIII of the Periodic Table of the Elements which have been modified with phosphine ligands are used. The metals are preferably chosen from the group consisting of Ru, Os, Pd, Pt, Rh and Ir. In particular, the metals are chosen from the group consisting of Rh and Ir.
The type of phosphine ligands which are used varies depending on the method by which the process according to the invention is carried out. This process can be carried out homogeneously in an organic phase, optionally with the addition of a solvent, heterogeneously in organic phase using an insoluble catalyst fixed to a support, optionally with the addition of an organic solvent, or in a two-phase system having one aqueous phase and one organic phase, optionally with the addition of an organic solvent.
If the homogeneous reaction method is chosen, the customary organophosphines are generally suitable; these may be mono-, bi- or else polydentate. In general, mono- or bidentate phosphines are used. These may be chosen from the known organophosphines soluble in the customary solvents. These are, for example, triarylphosphines, trialkylphosphines and alkylene- and arylene-bridged diphosphines which carry alkyl or aryl substituents. Examples include trimethylphosphine, triisopropylphosphine, tricyclohexylphosphine, triphenylphosphine, diphenylphosphinoethane and -methane, dimethylphosphinoethane and -methane, and the phosphines known under the name BINAP.
If the abovementioned phosphines are used, the reaction is carried out without the addition of a solvent or with the addition of a customary solvent, for example heptane, toluene, diethyl ether, dioxane or methanol or else mixtures thereof.
The resulting carboxylic acid is isolated by, for example, removing it from the reaction mixture by distillation, or extracting it therefrom, for example by acid-base extraction or using a suitable solvent.
Simple isolation and separation of the product from the catalyst is, according to one variant of the present invention, possible using phosphine ligands fixed to supports. All of the abovementioned phosphine ligands suitable for use in the process according to the invention can be fixed to suitable supports. These are, firstly, organic polymers, for example polystyrene, which may optionally be modified (Merrifield resin, Wang resin, aminomethyl-substituted polystyrene), Tentagel and polyamide resins. It is also possible to use inorganic supports, such as silicon dioxide and pulp.
In this process variant in which organophosphines fixed to supports are used, the reaction mixture can, when the reaction is complete, be separated off simply by decantation from the catalyst, which can then be reintroduced into the reaction. The reaction mixture separated off from the catalyst is then isolated and purified using customary methods, for example removal of the solvent by distillation followed by purification of the product by distillation.
In a preferred embodiment of the present invention, the process is carried out in a water/organic solvent two-phase system, in which case water-soluble phosphine ligands are used. These phosphine ligands may be mono- or bidentate and have the customary organic groups known to a person skilled in the art on the organic substituents bonded to the phosphorous, as a result of which the solubility in water is effected. Examples of such groups which effect solubility in water include carboxyl functions, hydroxyl functions, alkoxylated hydroxyl functions, phosphonato functions and sulfonyl functions, preferably sulfonyl functions.
One group of suitable water-soluble phosphine ligands are the triarylphosphines corresponding to the formula (I) below 
in which Ar is a phenyl or naphthyl radical, M1, M2 and M3, independently of one another, are an alkali metal ion, an optionally organosubstituted ammonium ion, an alkaline earth metal ion or a zinc ion and are present in the stoichiometrically required amount, and x, y and z are identical or different and independently of one another are 0 or 1.
Preferably, in formula (I), x, y and z are 1, and Ar is a phenyl radical. In particular, trisodium tri(n-sulfonyl)phosphine (TPPTS) is used as ligand according to formula (I).
Another group of suitable substituents corresponds to the formulae (IIa) or (IIb) 
in which M1 to M6 or M1 to M8, respectively, independently of one another are an alkali metal ion, an optionally organosubstituted ammonium ion, an alkaline earth metal ion or a zinc ion, and are present in the stoichiometrically required amounts, and a-f and a-h, respectively, independently of one another are 0 or 1.
Preferred phosphines of the formula (IIa) are those in which 3 to 6 sulfonyl groups SO3M are present; preferred phosphines according to the formula (IIb) have 4 to 8 sulfonyl groups. Of the phosphines corresponding to the formulae (IIa) and (IIb), particular preference is given to the ligands known under the name BISBIS and BINAS.
If the reaction is carried out in a two-phase system, an organic solvent may be present, but the reaction may also be carried out in the absence of a solvent. Examples of suitable organic solvents include nonpolar solvents, such as paraffins, aromatic solvents, for example toluene and xylene, ethers, such as, for example, diethyl ether and methyl tert-butyl ether, acetonitrile and chlorinated hydrocarbons, such as, for example, dichloromethane and chloroform.
The catalytically active catalyst complex is prepared by generally known methods, generally by mixing a suitable precursor compound with the respective phosphine ligands in the required amounts. Suitable precursor compounds are known to the person skilled in the art. Suitable rhodium precursor compounds include, for example, RhCl3.3H2O, [Rh(COD)Cl]2(COD=1,5-cyclooctadiene) and Rh(CH3COO)3 and other compounds known to the person skilled in the art.
Examples of suitable iridium precursor compounds which may be mentioned are IrCl63xe2x88x92, IrCl3.n H2O and H2[IrCl6].n H2O.
The phosphine ligands used in the process according to the invention are used in all process variants in relative amounts with regard to the rhodium or iridium metal which are at values of from 1:3 to 1:1000, preferably 1:10 to 1:100. The catalyst is used in an amount which is at values of from 100 to 10000, preferably 500 to 10000, mol of lactone/mol of metal ion.
In the preferred process variant according to the present invention, in which the water-soluble phosphines (I), (IIa) or (IIb) are used, the metal/phosphine ligand ratio is 1:3 to 1:1000, preferably 10 to 100. The aqueous phase comprises here 20 to 5000 ppm of metal ion, preferably 250 to 1500 ppm. The relative amount of metal ions which is used is 1xc2x710xe2x88x925 to 1xc2x710xe2x88x922 mol of metal ion/mol of lactone. The aqueous phase comprises here 1 to 25% by weight, preferably 2.5 to 15% by weight, of phosphine ligand.
The process according to the invention is carried out in all process variants at temperatures of from 50 to 200xc2x0 C. If functional groups are present on the lactone, the temperature is preferably 50 to 150xc2x0 C., in particular 70 to 130xc2x0 C., and the process is carried out under a hydrogen atmosphere at pressures of from 1 to 150 bar, preferably 1 to 30 bar.
The product mixture obtained following cleavage of the lactone and which has the carboxylic acids formed is separated from the solution which comprises the catalytically active metal complex or the phosphine ligands. In the case of a homogeneous reaction method in the organic phase, this is achieved by distilling off the solvent and distilling the residue containing the product, optionally following oxidation of the phosphine ligands or converting them into a phosphonium salt. If an immobilized ligand is used, the product solution is removed from this by decantation and subsequently worked up in an appropriate manner.
If the process is carried out as a two-phase reaction, the work-up comprises, firstly, simply separating off the organic phase which contains the product from the aqueous phase. The carboxylic acid is then purified by the known methods, conventionally by distillation. The aqueous catalyst solution can then be reused.
The process of lactone cleavage according to the invention can be used widely and is suitable both for xcex3-lactone and for xcex4-lactone. It is possible to use lactones which have different substituents on their ring system or else themselves have a double bond in the ring. Examples of functional groups which can have the lactone substrate include olefinic double bonds and acetylenic triple bonds, carboxyl functions, carbonyl functions, hydroxyl functions, epoxide functions, nitrile groups, amino groups, nitro groups, in particular olefinic double bonds.
Depending on the reaction conditions chosen a double-bond isomerization can be observed. Nonobservance of the respective reaction conditions required for the reaction to proceed gently to attain only the desired lactone cleavage, may also lead to the observance of a more or less complete hydrogenation of the functional groups, such as, for example, of the olefinic double bonds.
As already mentioned, one of the advantages of the process according to the invention is that functional groups are retained during the lactone cleavage. In particular, the process of lactone cleavage according to the invention can be readily carried out such that no olefinic double bonds are hydrogenated. The process is therefore suitable in particular for the cleavage of lactones which have olefinic double bonds on the substituents or an olefinic double bond in the ring itself.
The process of lactone cleavage according to the invention is particularly suitable for converting 2-ethylidene-6-hepten-5-olide into 2-ethylidene-6-heptenoic acid or isomers thereof. Here, it is possible to achieve conversions of 100% and selectivities for the 2-ethylidene-6-heptenoic acid or for double-bond isomers which form as a result of rearrangement reactions of up to 100%.
The present application also refers to a process for the preparation of 2-ethylheptanoic acids by cleavage of 2-ethylidene-6-hepten-5-olide and hydrogenation of the resulting 2-ethylidene-6-heptenecarboxylic acid or isomers thereof. 2-Ethylheptanoic acid is an interesting alternative to 2-ethylhexanoic acid. The latter is used as a starting material for lubricants, plasticizers and alkyd resins. The acid is used here, depending on the intended use, in the form of its esters, metal salts or the acid itself. The acid is frequently also converted to the corresponding alcohol by hydrogenation, which alcohol is esterified with certain carboxylic acids and then used as plasticizer.
However, in view of the preparation aspects, 2-ethylheptanoic acid is advantageous since it can be prepared from two C4 building blocks and one C1 building block (butadiene and CO2 respectively). Due to the uneven number of carbons, this is not the case for 2-ethylhexanoic acid, and recourse has to be made to propene for its preparation. However, propene is a raw material in short supply compared with butadiene, meaning that it is desirable to be able to use the latter as a raw material.
2-Ethylheptanoic acid as a C9 acid is known per se. It has properties which predestine it as replacement for 2-ethylhexanoic acid which has hitherto been used on a large scale. However, despite the number of carbon atoms which is favorable per se, it has not hitherto been possible to prepare 2-ethylheptanoic acid in a manner which is cost-effective and which permits synthesis on an industrial scale.
It was a further object of the present invention to provide a process with which 2-ethylheptanoic acid can be prepared simply, cost-effectively and in high yields. The process should in addition be able to be operated on an industrial scale.
This object is achieved by a process for the preparation of 2-ethylheptanoic acids, which comprises the above-described catalytic cleavage of 2-ethylidene-6-hepten-5-olide to give 2-ethylidene-6-heptenoic acids and isomers thereof and the hydrogenation of this carboxylic acid or of the resulting isomer mixture.
According to one embodiment of the present invention, the lactone cleavage and the hydrogenation of the olefinic double bond can be carried out in a single process step. Here, the catalyst used in the lactone cleavage is chosen and the reaction conditions are adjusted such that the reaction is not complete following cleavage of the lactone ring, but the olefinic double bonds in the substrate are likewise hydrogenated. This hydrogenation can be achieved, in particular, using severe reaction conditions, for example temperatures of  greater than 125xc2x0 C. and pressures of  greater than 10 bar. According to another preferred embodiment of the present invention, the hydrogenation is carried out using a customary catalyst system known per se over the ethylidene-heptenecarboxylic acid isomer mixture, formed following lactone cleavage, which has been freed from the catalyst system used.
The hydrogenation is carried out using the suitable catalyst compounds known to a person skilled in the art, it being possible for the hydrogenation to be carried out homogeneously or heterogeneously. The hydrogenation is preferably carried out under heterogeneous conditions. In this heterogeneous method, it is preferred to use, as catalyst, metals chosen from the group consisting of nickel, palladium and platinum. Mixtures of these preferred metals can also be used. The catalyst metals or mixtures thereof can be used without support material. If a support material is used, then it consists of the customary materials known to a person skilled in the art, for example activated carbon, Al2O3, SiO2, ZrO2 and MgO, preferably activated carbon or Al2O3.
During the hydrogenation of the ethylidene-heptanecarboxylic acids, it is possible for an organic solvent to be present. Examples of suitable organic solvents include lower alcohols, paraffins, ethers. The reaction can, however, also be carried out in the absence of a solvent. The chosen temperatures are from 0 to 300xc2x0 C., preferably 40 to 220xc2x0 C., and the pressures are from 1 to 300 bar, preferably 5 to 15 bar.
In this way, complete hydrogenation of the olefinic double bond of the ethylideneheptenoic acids used can be achieved.
The application is now described in more detail in the examples below.